A bit ago I had a deviant interested in making her (or his) own erotica, but she (or he) was running into this issue: they wanted it to feel deep, to feel like it had substance. They were asking me about it, and I marked it as something I wanted to respond to, but thought it might be nice to respond to it in a journal entry.
A few things to consider when making your erotica:
1.) If you want a piece of erotica that has substance, it will follow much of the advice for writing in general. Things like...
A.) Make sure no one is exactly as they seem. People who are exactly what they show to the world... that's a rarity. And boring. You want to surprise people. You want to make the person who seemed trustworthy... dangerous. You want to make the person who seemed dangerous... show their compassion. That's interesting, and that will give your piece interest.
B.) Flip between success and failure, good and bad, in scenes. Stories where characters experience nothing but success in their endeavors... again, it gets repetitive. It takes all the surprise, all the interest out of the work. Nothing but failure, nihilistic. For example: Samus finds a screw and frees herself (success), but when she tries to jump out of the cage she trips and falls into the lava (failure)
C.) Change. That's the point of any the majority of good stories. You want the character to be arcing from something toward something. In Material Girl Noah arcs from being callous to the plight of others to becoming not just aware, but empathic towards others.
2.) Use your work to explore a problem, something you're afraid of, or a tension in your real life.
All art is metaphor, or a tool, for you to explore tension and issues in your real life. Material Girl was my process of exploring how my friends, family, romantic interests, and the world at large, would respond if I was spontaneously forced to transition. It was something I was struggling with in real life, and I used my art to explore it. By using metaphor, by making your "Gender" into a Ribbon or abstracting it, you can get some distance on it and give it some real thought.
So find the things that engage you, and probe those ideas. Rape fantasies "tie in" to feelings of wanting to be desirable (on the part of being taken) and addressing fears of inadequacy (on the part of the one doing the taking). Vore connects with consumerism as well as feelings of loving something so much you could "literally eat you up". Use the project, the art, as a platform for you to explore your own feelings, experiences and thoughts on the issue and the piece will resonate.
3.) Give thought to the girl.
Because it so often IS the girl who is being reduced to an image, her body parts, etc. I think it's easier for me, being a girl, to do this. It gives me a bit of an edge. But anyone with an ounce of empathy can do this. What is her reaction to the situation? How is this serving her needs? Imagine the women you know in your life and think: if they were in the situation you're depicting, how would they react? If Zelda is tied up and forced to dress as Gerudo, is she (as Zelda) Irate? Embarrassed? Does the demon being pulled into the angel's sexual "sanctuary"... is she resisting a temptation that she secretly wants? How might this encounter tie into her larger context in terms of her life, her relationships, her profession. Give it consideration.
That's about all I can say on the subject. Think about it like you'd think about any good story, use it to explore yourself and the world, and give the object of desire consideration. If anyone has any specific questions, I'll do my best to respond or provide examples of what I'm talking about. Otherwise, go forth and make great erotica!